Such methods and apparatus are generally known. They serve for the detection of hot spots. Hot spots are areas on the solar cell in which an excessive current is flowing due to a local short circuit, which translates into heat so that the result may be overheating of the solar cell. In the known method, an infrared camera is first used to measure a distribution of radiation emitted on the solar cell surface area. Then the solar cell is illuminated, or a voltage or current is applied to the solar cell, which allows the solar cell to be locally heated. The current or voltage flowing through the solar cell is measured and recorded as a function of the illumination and/or the applied voltage and current, respectively, i.e. a so-called characteristic of the solar cell is measured. Subsequently, a distribution, resulting from the characteristic, of radiation emitted on the solar cell surface area is measured, and a difference distribution is produced from the two measured distributions. Certain properties of the solar cell can be calculated from the difference distribution, such as a maximum radiation intensity or a surface area of the maximum radiation intensity.
The drawback of the well-known methods is that the energy introduced into the solar cell due to measuring the characteristic varies from one solar cell to another since different solar cells heat up to differing degrees under different applied voltages or currents. When sorted, solar cells having a radiation intensity above a limit value are rejected. However the amount of energy introduced into a shaded solar cell is not taken into consideration when a certain voltage is applied to the solar cell.